This invention relates generally to alpha-alumina articles and, more particularly, to alpha-alumina articles made using a sol-gel process.
Porous ceramics having interconnected pores ranging in size from nanometers to millimeters have been used as filters, catalyst carriers, acoustic absorbers, membranes, and heat exchangers in various industrial applications. Alumina, silica, mullite, corderite, aluminosilicate, and zeolites are among the commonly used ceramic materials.
Porous alumina is an excellent candidate for many applications, because of its good mechanical strength, thermal stability, and chemical durability. Hollow alumina honeycombs are used in food and beverage processing and in biotechnology for purposes such as removing bacteria in breweries and filtering gases and fluids. The same material also can be used to remove sulfur and nitrogen from coal gasifiers. Alumina materials having small pore sizes also are used as molecular sieves to separate hydrogen from coal gasification gases. Other applications of porous alumina include filters for high temperature gas clean-up and catalyst support for removing NOx and SOx from flue gases. Recently, porous alumina has been used as a casting mold for slip casting processes.
In a particularly important application, porous alumina substrates are used as the diffusion rate-determining member in thin-film, limiting-current type oxygen sensors for both low and high oxygen concentrations. In such applications, the alumina substrate serves as a diffusion barrier for oxygen transport. When the admission of oxygen is restricted, such as by the porosity and pore size of the substrate, a saturated region is reached in which the sensor output current remains constant despite a voltage increase. This phenomenon occurs because of the rate-determining oxygen transport process from the outside environment, through the porous substrate, and onto the sensor electrode. Consequently, a porous (sintered) alumina substrate can be used effectively as the rate-determining member for the oxygen diffusion process.
Currently, such porous alumina substrates typically are made by tape casting of alumina slurries that incorporate alumina powders of several different particle sizes. This casting process generally leads to an inhomogeneous microstructure in the substrate, with low manufacturing yields, especially for sensor applications. Other disadvantages of the tape casting process include large pore sizes in the substrates and uneven pore size distribution. For the oxygen sensor applications described above, large pore sizes are detrimental because they are outside the Knudsen diffusion regime, causing: (1) loss of linearity between current and concentration at high oxygen concentrations, (2) limited low oxygen concentration detection capabilities (detection lower limit xcx9c100 ppm at xcx9c50 nm pore size, 50 ppm at 30 nm pore size), and (3) slow response time.
The sol-gel process is a well-known technique for making ceramic materials in varying forms such as thin film, bulk, fiber and powder. Boehmite (Alxe2x80x94Oxe2x80x94Oxe2x80x94H) and pseudoboehmite are good precursors for the fabrication of alpha-alumina-based ceramics. Sol-gel processing with boehmite provides better chemical homogeneity and improved microstructure control. When heated to high temperatures, boehmite transforms to several transition aluminas before the formation of the thermodynamically stable alpha phase, at about 1200xc2x0 C.
Monolithic alumina also has been made by hydrolyzing aluminum alkoxides, such as aluminum sec-butoxide, at 90xc2x0 C. Although this process has produced transparent monolithic boehmite gels having small pore sizes and a narrow pore size distribution, the densities of these gels have been unduly low after sintering, i.e., less than a 45% theoretical density, or greater than a 55% porosity.
Boehmite gels of high density originally were considered impossible to obtain at sintering temperatures below 1600xc2x0 C. However, by seeding the boehmite with alpha-alumina powders, the seed particles can function as nucleation sites that increase the transformation kinetics and decrease the required transformation temperature. Seeded boehmite gel-derived alpha alumina substrates can be sintered to a density of about 99% at temperatures as low as 1150xc2x0 C. However, monolithic alpha-alumina gels generally have not been obtained directly from gels in this manner, because the gels generally crack during drying. This cracking problem has restricted the development of alumina gels to small dimensions, such as thin-film coatings and abrasive grains.
Thus, although seeding has allowed boehmite gels to be sintered to a high density, the fabrication of alumina monoliths generally has required the cracked gel fragments first to be ground into powder and then pressed into pellets, for subsequent consolidation into dense compact forms before sintering. Therefore, a powder-dispersion-consolidation process still has been required to fabricate dense alumina monoliths.
It should therefore be appreciated that there is a need for a practical method for producing monolithic porous alumina articles having small, well-defined pore radii. The present invention fulfills this need.
The present invention resides in a high-density, crack-free monolithic alpha-alumina article, and a sol-gel process for making it, having small, well-defined pore diameters. The process includes: (1) casting in a mold a solution of an aluminum alkoxide (e.g., aluminum sec-butoxide), alpha-alumina powder, water, and a peptizing agent, (2) drying the cast solution in a controlled environment having a temperature in the range of about 25 to 40xc2x0 C. and a humidity in the range of about 75 to 95%, to produce a dried gel, and (3) sintering the dried gel, to produce the high-density, crack-free monolithic alpha-alumina article.
In a separate feature of the invention, the process uses a mold formed of a material selected from the group consisting of PMP (polymethylpentene), Teflon PFA (perfluoroalkoxy resin), Teflon FEP (fluorinated ethylenepropylene copolymer), and Teflon PTFE (poly tetrafluoroethylene polymer). The entire mold, or alternatively merely an inner liner of the mold, can be formed of such material. Further, sintering occurs at a temperature in the range of about 1000xc2x0 C. to about 1400xc2x0 C., and preferably in the range of about 1000xc2x0 C. to about 1100xc2x0 C., for a time period in the range of about 3 to about 12 hours. In addition, casting can include preliminarily applying a mold release agent, preferably a silicone agent, to the mold. Casting and drying can occur using the same mold.
The resulting high-density, crack-free monolithic alpha-alumina article has a density in the range of about 83 to 98%, with pores having diameters in the range of about 8 to 120 nanometers. The article, thereby, is suitable for use as a substrate for such devices as a gas sensor device, e.g., an oxygen sensor device. The preferred article has a density greater than about 95%, with pores having an average radius of less than about 30 nanometers. In addition, the article preferably has the shape of a disk, with a diameter greater than about 25 millimeters.
Other features and advantages of the present invention should become apparent from the following detailed description of the invention, which discloses, by way of example, the principles of the invention.
The present invention resides in a sol-gel process for making high-density porous alpha-alumina articles, of theoretical density in the range of about 83 to 98%. The gels are produced by casting a suitable aluminum alkoxide (e.g., aluminum sec-butoxide or aluminum iso-propoxide), alpha-alumina, water, and a suitable peptizing agent (e.g., nitric acid, hydrochloride acid, or ammonium hydroxide). Cracking of the gels during drying is prevented by controlling the temperature and humidity of the drying condition and by using a mold formed of PMP (polymethylpentene), Teflon PFA (perfluoroalkoxy resin), Teflon FEP (fluorinated ethylene propylene copolymer), and Teflon PTFE (poly tetrafluoroethylene polymer), with a silicone release agent (e.g., Leco, Part No.811-271).
Optimization of the drying humidity and temperature reduces the internal stress of the gel during drying and assists in the formation of a crack-free monolithic gel. In addition, the dried gels can be fired to high temperatures for the formation of stable aluminum oxides without cracking. The drying conditions include a controlled temperature of about 25 to 40xc2x0 C. and a controlled humidity of about 75 to 95% relative humidity (RH), for about 70 to 100 hours. The gels are then further dried at room temperature conditions, i.e., about 25xc2x0 C. and about 55% RH, for another 24 to 72 hours. Suitable drying conditions for preventing cracking during gelation are disclosed in the following Table 1.
As can be seen in Table 1, it is desirable to maintain the temperature in the range of about 25 to 40xc2x0 C., while at the same time maintaining the relative humidity in the range of about 75 to 95%. If the initial drying conditions are outside of these limits, the monolithic gels can crack.
Another feature of the process of the invention is the use of special materials for the molds. The use of a PMP or Teflon mold, and the use of a silicone release agent, allows the gel to be readily separated from the mold after gelation without cracking. PMP molds having an opening diameter of 63.5 mm and a volume of 125 ml can be obtained from commercial sources, e.g., Fischer Scientific, Part No. 118.2330. Teflon molds of various types (PTFE, FEP, PFA) can be obtained from numerous commercial sources or produced to custom requirements.
The silicone release agent preferably is sprayed onto the interior wall of the mold about 10 to 60 minutes before sol casting. The sprayed agent should coat a uniform layer on the mold wall without the formation of liquid droplets. Excess liquid remaining on the mold can be removed by using a lint-free laboratory tissue. A non-uniform coating of the wall with the release agent can result in an uneven release of the gel from the wall, whereas excess droplets of release agent can cause dimples to be formed in the dried gels.
The benefits of using the specified mold material along with a silicone release agent are demonstrated in Table 2. It is seen that the dried monolithic gel would crack unless the mold or its inner liner is formed of some type of PMP or Teflon and a silicone release agent is used. Molds formed of certain materials such as glass or polystyrene produced cracked gels even if a silicone release agent was used. PMP and Teflon molds produced cracked gels if used without a silicone release agent, but they provided good gels when used in combination with such a release agent. A Teflon liner coated onto a metal mold also produced good articles when used with a silicone release agent.